Liquid ink writing instruments exist in two categories. In the first category, the reservoir which contains the ink is isolated from ambient air and does not contain air; a flexible piston moves in the reservoir as the ink is consumed and provides sealing.
In writing instruments of the second category, the reservoir which contains the ink is not isolated from ambient air; in particular, the capillary element penetrates into the ink reservoir so that ink is fed continuously to the writing head. When the writing instrument is in use, the ink which is deposited on the medium is replaced as it is consumed by ink from the reservoir diffusing into the capillary element and thence to the writing head, with the ink which leaves the reservoir being replaced by air which passes through the porous capillary element. If conditions of use change, and, in particular, if the pressure of the air contained in the reservoir changes due to a rise in air temperature, then an abnormal amount of ink flows into the capillary transfer element and can give rise to blots or smudging when the cap is taken off the instrument. To avoid that phenomenon, that type of instrument is fitted with a member generally referred to as a “buffer reservoir” having the function of absorbing excess ink coming from the reservoir before it reaches the writing head, and once normal conditions are re-established, for returning the ink it contains to the capillary element. One such writing instrument is disclosed in particular in document EP 0 516 538.
In instruments of the first category, the transfer of ink from the ink reservoir to the writing head is not continuous. This transfer takes place progressively as ink is consumed by the writing head while the instrument is in use due to the difference in pressure that such consumption of ink generates inside the body of the instrument. One such solution is known in particular from document EP 0 240 994, related to U.S. Pat. No. 4,973,180, which implements a valve mechanism that is interposed between the main ink reservoir and a secondary ink reservoir into which the capillary element penetrates. The valve mechanism may comprise, in particular, a diaphragm of elastic material such as a silicone rubber, the diaphragm being provided with a slot such that when a predetermined pressure difference acts on said diaphragm, the slot opens and ink is fed from the main reservoir to the secondary reservoir. In operation, while the instrument is in use, the consumption which occurs of the ink in the capillary element causes the pressure in the secondary reservoir to be reduced, thereby causing the valve to be opened, with ink flowing into the secondary reservoir and the capillary element being resupplied with ink. To achieve such operation, it will be understood that the pressure difference enabling the valve to be opened is relatively small. In document EP 0 240 994 which describes such an instrument, the pressure difference is said to be of the order of 125 millimeters (mm) of water column. It should be emphasized that in instruments of that type, the problem of air in the main reservoir heating does not arise since the main ink-containing reservoir is isolated from ambient air and does not contain any air, having a flexible piston which moves inside the reservoir as the ink is consumed. The presence of such a flexible piston makes such an instrument complex to manufacture.
It therefore would be desirable to provide a liquid ink writing instrument which does not have the flexible piston of document EP 0 240 994 and which is not necessarily fitted with a buffer reservoir as described in document EP 0 516 538.